This invention relates to a method of separating salt from polyglyceride-containing heavy ends obtained, for example, in conventional manufacture of glycerine via hydrolysis of epichlorohydrin or glycerol dichlorohydrins. More particularly, this invention is directed to a solvent-aided crystallization process for separation of salt from glycerine foots still bottoms, a heavy ends waste stream, obtained in glycerine manufacture, containing salt and polyglycerides as principal components along with a minor amount of water.
The conventional process for manufacture of synthetic glycerine by hydrolysis of epichlorohydrin or glycerol dichlorohydrins affords a crude reaction product which contains significant amounts of salt (sodium chloride) and polyglycerides (polyglycols) in addition to glycerine and water. In this process, salt is typically obtained as a primary reaction product since sodium hydroxide and/or a basic sodium salt e.g. sodium carbonate, is usually employed to promote the hydrolysis reactions. The polyglycerides, in turn, are obtained from a variety of sources--e.g., polymerization of reactant epichlorohydrin, glycidol intermediate and/or side reactions involving glycerine--which combine to make these polyglycerides or polyglycols the most significant by-product formed in the process. While conventional techniques for purification and recovery of this crude glycerine include a series of concentration i.e. distillation, steps wherein salt is removed from the glycerine product via precipitation and solids separation, including water washing, it is not possible on a practical scale to completely eliminate the salt from the glycerine product by these concentration techniques alone. As a result, complete product recovery in the typical purification scheme invariably involves a high temperature distillation step wherein at least a portion of the glycerine product is distilled overhead from a salt and heavy ends i.e. polyglyceride, bottoms product. This bottoms product, known conventionally as the glycerine foots bottoms or foots still bottoms typically contains from about 50 to 65% by weight salt, from about 50 to about 35% by weight polyglycerides, in addition to small amounts of unrecovered glycerine and other organic impurities. In the past, much of this glycerine foots still bottoms was disposed of by passing it directly to conventional biotreatment facilities since it was considered impractical to further isolate and/or recover the various components in view of their low value and the corresponding problems and expense attendant to separation of such a complex and difficult to separate mixture. However, the glycerine foots still bottoms are not readily biodegradable and as a result they contribute disproportionately to the biotreater load. Further, alternative direct disposal methods such as incineration are not acceptable because the high proportion of inorganic matter (salt) in the foots still bottoms would likely give rise to excessive particulate formation in the incineration off-gases in the absence of expensive scrubbing facilities.
From the foregoing it is apparent that considerable advantage would be obtained if an economic and effective technique could be developed for substantially complete separation of the glycerine foots still bottoms into its major components, i.e., separation of salt from the organic polyglyceride phase. Such a separation would then allow each of the components to be disposed of in the simplest and most environmentally acceptable way i.e., incineration of the organic polyglyceride phase and biotreatment of the salt, without encountering problems attendant to the use of similar means for direct disposal of the mixed waste stream.